Chlorine and caustic are essential and large volume commodities which are basic chemicals required in all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of an alkali metal chloride with a major portion of such production coming from diaphragm type electrolytic cells. In the diaphragm electrolytic cell process, brine (sodium chloride solution) is fed continuously to the anode compartment and flows through a diaphragm usually made of asbestos, backed by a cathode. To minimize back migration of the hydroxide ions, the flow rate is always maintained in excess of the conversion rate so that the resulting catholyte solution has unused alkali metal chloride present. Hydrogen ions are discharged from the solution at the cathode in the form of hydrogen gas. The catholyte solution, containing caustic soda (sodium hydroxide), unreacted sodium chloride and other impurties, must then be concentrated and purified to obtain a marketable sodium hydroxide commodity and sodium chloride which can be reused in the chlorine and caustic electrolytic cell for further production of sodium hydroxide.
With the advent of technological advances such as the dimensionally stable anode and various coating compostions therefor which permit ever narrowing gaps between the electrodes, the electrolytic cell has become more efficient in that the current efficiency is greatly enhanced by the use of these electrodes. Also, the hydraulically impermeable membrane has added a great deal to the use of electrolytic cells in terms of the selective migration of various ions across the membrane so as to exclude contaminants from the resultant products thereby eliminating some costly purification and concentration steps of processing.
The dimensionally stable anode is today being used by a large number of chlorine and caustic producers but the extensive commercial use of hydraulically impermeable membranes has yet to be realized. This is at least in part due to the fact that a good, economical electrolytic cell structure for use of the planar membrane versus the three dimensional diaphragm has yet to be provided. The geometry of the diaphragm electrolytic cell's structure makes it undesirable to place a planar membrane between the electrodes, hence the filter press electrolytic cell structure has been proposed as an alternative electrolytic cell structure for the use of membrane in the production of chlorine, alkali metal hydroxides and hydrogen.
There are two basic types of electrochemical cells commonly used for the electrolysis of brine solutions to form chlorine and caustic, i.e., monopolar cells and bipolar cells. Although bipolar cells are not the subject of the present invention, it is helpful to understand the operation of bipolar cells to fully comprehend the prior art.
A bipolar filter press electrolytic cell is a cell consisting of several electrochemical units in series, as in a filter press, in which each unit, except the two end units, acts as an anode on one side and a cathode on the other, with the space between these bipolar units being divided into an anode and a cathode compartment by a membrane. In a typical operation, an alkali metal halide solution is fed into the anode compartment where halogen gas is generated at the anode. Alkali metal ions are selectively transported through the membrane into the cathode compartment and associate with hydroxide ions at the cathode to form alkali metal hydroxides, as hydrogen is liberated. In this type of cell the resultant alkali metal hydroxide is significantly purer and can be more concentrated, thus minimizing an expensive evaporation and salt separation step of processing. Cells where the bipolar electrodes and membranes are sandwiched into a filter press type construction are electrically connected in series, with the anode of one, connected to the cathode of an adjoining cell through a common structural member of some sort.
Monopolar, filter press, electrolytic units are known comprising terminal cells and a plurality of cathode units and anode units positioned alternately betwee the terminal cells. A separator, which may be a diaphragm, or an ion exchange membrane, is positioned between each adjacent anode and cathode to divide the cell into a plurality of anode and cathode units. Each of the anode units is equipped with an inlet through which electrolyte may be fed to the unit and an outlet or outlets through which liquids and gases may be removed from the unit. Each cathode unit is similarly equipped with an outlet or outlets and if necessary with an inlet through which liquid, e.g. water, may be fed to the cathode units. Each of the anodes in the cell is also equipped with connections through which electrical current may be fed to the cell and each of the cathodes is equipped with connections through which electrical current may fow away from the cell.
In monopolar cells, electrical current is fed to one electrode unit and removed from an adjacent, oppositely charged unit. The current does not flow through a series of electrodes from one end of a series of cells to the other end of the series, as in a bipolar cell assembly.
To assure the effective use of substantially all of the surface of the electrodes in a monopolar cell, it is desirable to provide electrical current to the electrode relatively evenly and without excessive resistance losses. To accomplish this, workers in the prior art have devised a variety of mechanisms and designs by which electrical current may be efficiently deivered to the electrode.
The first, and most obvious means to provide electrical current to a monopolar cell is by directly connecting the power supply to the electrode using a wire, cable, rod, etc. Although this design minimizes the resistance losses in the electrical distribution system, it does not work well because some electrodes are not sufficiently electrically conductive to distribute the electrical current relatively uniformly throughout the entire electrode body. This is particularly true for titanium electrodes, which are frequently used in chlor-alkali cells. Thus, it is frequently necessary to provide a plurality of connections to the electrode to assure proper current distribution.
U.S. Pat. No. 4,464,242, for example, provides a thin, rectangular sheet electrode structure having electrical connections all across one, long edge. The electrode structure is sufficiently electrically conductive to distribute the electrical current through a narrow width of the electrode but not sufficiently conductive to distribute electrical current through the length of the electrode. Obviously, this electrical distribution means, works only for narrow electrodes and is not suitable for larger electrodes. In addition, the system is cumbersome and expensive because so many electrical connections are involved.
In a similar manner, U.S. Pat. No. 4,464,243 shows a cell where a plurality of electrode strips are electrically connected at their ends to an electrically conductive hollow frame. Since some electrodes are not very electrically conductive, the height of the electrodes is limited and such a system is limited to shorter electrodes. Also, this means of electrical attachment involves a plurality of electrical connections, each of which is an actual or potential electrical discontinuity. U.S. Pat. No. 4,464,243 also shows electrode sheets having ridges wherein the sheet acts as the conductor.
An alternate means for distributing electrical current to monopolar electrodes is illustrated in U.S. Pat. No. 4,056,458 where a plurality of titanium coated copper rods extend vertically between a pair of parallel, planar electrodes. The rods are electrically connected to both of the electrodes and provide electrical energy thereto. Because the rods are positioned at frequent intervals, the electrical current does not have very far to travel through the electrodes and the overall dimensions of the electrodes may be increased, so long as the number of rods is correspondingly increased. This means of electrical connection is, however, not entirely satisfactory because of its expense and complexity. In addition, there are a large number of actual or potential electrical discontinuity sites.
An electrical distribution means for monopolar electrochemical cells having a minimum number of parts, a minimum number of electrical connections, employing inexpensive, readily-available materials and allowing the use of electrodes of virtually any reasonable length and width would be highly desirable. It is the object of this invention to provide such a means.